The present invention relates to a fuel loading method for reactor cores, and more particularly to a fuel loading method suitable for a core made up by high burn-up fuels as well as to a reactor core constructed by such fuel loading method.
A core of a boiling water reactor is constructed by arraying fuel assemblies in the form of a rectangular lattice. Within the core, there are mixedly present fuel assemblies which are different in residence period (or the number of residence cycles) from one another. During each period of periodical inspection of a reactor, approximately 1/4 to 1/3 of total fuel assemblies (used fuel assemblies) in the core is usually replaced with fresh ones. Depending on the number of residence cycles, the fuel assemblies are also different in exposure and neutron infinite multiplication factor from one another. Accordingly, the fuel assemblies loaded in a core position of interest would have different powers, even if the fuel assemblies surrounding this fuel assembly are arrayed under the same conditions. In a usual core, to easily meet thermal limiting conditions for the core operation, a core unit cell is made up by four fuel assemblies adjacent to the position where a cross-shaped control rod is inserted, and those fuel assemblies making up each core unit cell are made different in the number of residence cycles from one another.
In conventional cores, it is general to load those fuel assemblies having the maximum number of residence cycles at the outermost periphery of the core, and load other fuel assemblies having the different numbers of residence cycles inside the outermost periphery of the core in an evenly dispersed manner. The resulting evened radial power distribution across the core is flat in an inner region of the core, but distorted to a large extent in an outer region of the core due to the presence of the core boundary. Accordingly, if the fuel assemblies arranged in the outer region of the core remain there, those fuel assemblies would be discharged out of the core before reaching a target degree of discharge exposure, with the result of poor fuel economy. In general, therefore, the fuel assemblies loaded in the outer region of the core are appropriately moved within the core (called fuel shuffling) so that all the fuel assemblies discharged out of the core may reach a target degree of discharge exposure as near as practicable. The fuel shuffling is performed during periodical inspection periods after shutdown of the reactor.
JP, A, 56-87891 discloses a conventional fuel loading method comtemplating that the core radial power distribution is distorted in the outer region of the core. In order that burnable poison contained in the fuel assemblies loaded in a circumferential zone of the core is completely burned up while leaving no burnable poison and the loss of excess reactivity is not caused, the disclosed prior art proposes to divide the core into a central zone and a circumferential zone, and load fresh fuel assemblies in the third layer from the outermost layer of the circumferential zone. After residing for one operation cycle, those fuel assemblies loaded in the third layer are moved to the second layer from the outermost layer for the purpose of making the core radial power distribution more flat.